It is well known to produce a foamed article by extrusion by feeding into an extruder a blend of a thermoplastic resin and a chemical blowing agent. This is called a chemical blowing method. In this method, initially the chemical blowing agent is thermally decomposed into gases within the extruder and then the resultant gases contribute to foam in the thermoplastic resin. Although this method is widely practiced, due to the products by the thermal decomposition other than gases, it has the following disadvantages; it is impossible to continuously operate the extruder for a long time because the products of the decomposition are formed on the surfaces of the screws, barrels, tips and dies of the extruder; the electrical properties of the foamed article tend to be deteriorated due to hygroscopicity of the products by the decomposition; and the produced foamed article tends to become discolored.
Meanwhile, it has been proposed to produce a foamed thermoplastic resin article by feeding into an extruder a blend of thermoplastic resin and a nucleating agent for foaming and forcibly injecting a foaming gas into the blend when it is melted within the extruder. This is called a gas foaming method. In this method, the gas injected into the molten resin grows into cells about the nucleating agent. With this method, there has been eliminated various influences due to the products by the decomposition of the foaming agent. However, this method has the difficulty to inject a constant quantity of gas into the molten resin always fluctuating in its pressure within the extruder, and therefore, to produce the foamed article having a uniform and constant expansion rate.